1. Field of the Invention
The present invention relates to the field of compression ignition engines.
2. Prior Art
Dual fuel compression ignition engines are well known in the prior art. Most commonly, such engines operate on a dual fuel consisting of diesel fuel and natural gas. The combination of a liquid fuel and a gaseous fuel had an advantage in the prior art in that diesel fuel could be injected in controlled amounts through a conventional diesel injector, and the gaseous fuel could be injected into the intake airstream. Typically such engines depend on the use of some diesel fuel to initiate compression ignition, with the gaseous fuel then supplementing the diesel fuel for increased power. Such engines frequently run on 100% diesel fuel at idle, with increasing amounts of gaseous fuel being included as the power requirements of the engine increase until running on perhaps 80% gaseous fuel at full power.
Diesel fuel is used with the gaseous fuel to obtain compression ignition, as the engines used were otherwise conventional diesel engines with no way to control compression ignition of the gaseous fuel by itself. However the use of injected diesel fuel for compression ignition worked satisfactorily, in that the self ignition temperature of the gaseous fuel typically is too high to self ignite in a conventional diesel engine, and further, if self ignition could be obtained, there would be no way to control the time of ignition, unless the gaseous fuel was injected into the combustion chamber at the appropriate time, rather than being mixed with the intake air.
A primary gaseous fuel of interest in the United States and many other places is natural gas, as it is plentiful and inexpensive. However natural gas has the disadvantage of not being liquifyable at ordinary temperatures, though does turn to a liquid at cryogenic temperatures. The use of a fuel that must be maintained at cryogenic temperatures in ordinary vehicles, such as passenger cars and trucks, is not practical. Accordingly for such use, natural gas must be stored in pressurized form, highly limiting the energy per unit of volume (energy density) of the compressed natural gas. Thus when used to power a vehicle, alone with spark ignition or in conjunction with sufficient diesel fuel for compression ignition in motor vehicles, the range of the vehicle is highly limited. Consequently such dual fuel engines are operated solely on diesel fuel, when necessary, when the compressed natural gas is fully used.
In U.S. Patent Application Publication No. 2007/0245982 published Oct. 25, 2007 and entitled “Low Emission High Performance Engines, Multiple Cylinder Engines and Operating Methods”, compression ignition engines and methods of operating compression ignition engines are disclosed which include, among other things, an engine operating cycle for use in camless engines whereby diesel fuel is injected into the combustion chamber early in the compression stroke without a conventional air intake stroke. Ignition temperatures are limited by a very limited air content in the combustion chamber, with additional air being injected after ignition to continue combustion to completely consume the fuel, and provide enough excess air (with or without some additional air intake) in the residual exhaust gas for the following combustion ignition while still limiting combustion chamber temperatures to below the temperature at which NOX is formed. The time of compression ignition is controlled by engine valve control, with cycle to cycle adjustments being made based on the time of ignition during the previous cycle. This operating cycle has a number of advantages for diesel fuel, including the advantage of fully vaporizing the diesel fuel in hot exhaust gases before combustion to avoid both hot spots and droplets that do not fully burn.